It is known to prepare materials capable of generating a sufficient amount of heat on contact with air to be able to be used in pyrotechnics. These materials are generally provided in a pulverulent form and are in particular inorganic compounds.
Among inorganic compounds, alloys and inter-metallic compounds can a priori offer good prospects. Calcium-based alloys are advantageous, for example: they are hard, brittle, can easily be detrimentally affected by water and exhibit exploitable melting temperatures. By way of illustration, the melting temperature of Ca.sub.2 Sn is 1120.degree. C. and that of CaSn.sub.3 is 627.degree. C.
Mention may also be made of calcium-magnesium alloys MgCa or Mg.sub.2 Ca for which the melting temperatures are 517 and 714.degree. C. respectively. Mention may further be made of calcium-zinc alloys, for example ZnCa for which the melting temperature is 385.degree. C.
The enthalpies of reaction (.DELTA.H) are as follows, assuming that the oxides are formed according to a reaction of the type ##EQU1##
______________________________________ 1 .DELTA.H (kcal .multidot. mol.sup.-1) ______________________________________ Ca.sub.2 Sn 2 -442 CaSn.sub.3 3.5 -568 Mg.sub.2 Ca 1.5 -449 MgCa 1 -300 ZnCa 1 -235 ______________________________________
Alloys based on sodium and on potassium have been widely studied for possible pyrotechnic applications. It has thus been possible to demonstrate that some of these alloys, for example the sodium-potassium alloy containing 50-80% by mass of potassium, may be potentially advantageous in pyrotechnics. This is because some of them react in the presence of oxygen or of water with a strong release of heat.
As regards the intermetallic compounds formed with sodium or potassium, their reaction with air is less violent that when Na or K are alone, which is per se a positive point.
Titanium forms a great many binary or ternary alloys with most of the elements of the periodic classification (nouveau traite de Chimie Minerale [New Treatise on Inorganic Chemistry], Paul Pascal, Masson).
Metallic titanium has the property of spontaneously oxidizing in the presence of oxygen, releasing a large amount of heat.
Moreover, much work has been carried out into the study of coruscative materials (this concerns intermetallic combined materials, that is to say that, after having reached the reaction temperature, these products are capable of releasing large amounts of energy).
Mention may be made, for example, of:
______________________________________ Ignition Reaction Enthalpy % temperature temperature measured Material by weight (.degree.C.) (.degree.C.) (J.g.sup.-1) ______________________________________ Ti Sb Pb 48 23 29 570 1010 1045 Ti Te 27 73 433 870 815 ______________________________________
Patents WO 089/10340 and U.S. Pat. No. 4,830,931 relate to processes for the activations of metal surfaces in order to render them pyrophoric.
The basic principle is the attack on the metal surface by a mixture of a metal and of its chloride, at a temperature such that the substrate remains in the solid state and that the mixture is in the liquid state. The reaction is long, since it lasts several tens of hours.
The metal, thus covered with an intermetallic layer, is subsequently activated by a sodium hydroxide solution.
The metal which has become pyrophoric must then be stored in a liquid of low volatility (examples: fluoroethane, nonane, glycerol).
However, it is expensive to prepare these compounds and the latter can thus be exploited industrially only to a fairly limited extent.
Compounds which have formed the subject of other studies in this field are metal pairs which are in a position, when the reaction temperature is reached, to release relatively large amounts of energy with the formation of an alloy.
It is thus necessary to transmit a sufficient amount of energy to these materials in order for the melting temperature of one of the metals of the pair to be reached and for the reaction to take place.
Explosive charges are generally used.
The exothermic reaction is brief.
However, these compounds are also expensive and, for this reason, can also be exploited industrially only to a limited extent.
Lithium-based alloys are also well known. Their high reactivity in the presence of moist air, for example, is known and is profusely described by J. C. Bailar, in "Comprehensive Inorganic Chemistry", Volume 1, 1973, 335-37, F. A. Cotton and G. Wilkinson, in "Advanced Inorganic Chemistry", 1972, 189-91 and F. E. Wang and M. A. Mitchell, in "J. Less Common Metals", 1978, 61, 237.
It is noted that organometallic compounds are often far removed from pyrotechnic applications. This is because they often exhibit the disadvantage of reacting violently in the presence of water. Now, the water content in air depends essentially on the weather conditions. It is therefore very clearly understood that the result of bringing these compounds into contact with air is uncertain and that, for this reason, these compounds are rather far removed from pyrotechnic applications.
In reality, in this field, there is interest in particular in compounds which give rise to reactions involving atmospheric oxygen (optionally nitrogen), in order for the product to retain an equivalent performance whatever the surrounding conditions.
It is a question of generating heat in the atmosphere by distributing therein a pulverulent product capable of spontaneously oxidizing. More specifically, it is a question of heating a volume of air (typically 1000 m.sup.3) while avoiding an excessively high temperature peak (so-called "thermal peak").
In this respect, tests have shown that the majority of the inorganic compounds indicated above exhibit in particular the following disadvantages:
their duration of combustion is too brief, PA1 the temperature reached is too high. PA1 it reacts instantaneously with air, that is to say in less than 0.5 second, PA1 its duration of combustion with air is greater than 5 seconds, PA1 its calorific value is high, PA1 its resistance to ageing is good, PA1 it is nontoxic, PA1 it is prepared by means of a process which can be readily industrialized, PA1 its cost is moderate, PA1 it does not give rise to a violent reaction when it is immersed in water. PA1 Z represents carbon or a polymer with a cyclic unit, in particular an aromatic unit, especially a polymer in which the basic unit is C.sub.6 H.sub.4 ; EQU 0&lt;x&lt;1, PA1 y being able to be an integer or a noninteger.
In addition, these compounds can be improved from the viewpoint of their intrinsic properties, for example their toxicity. Moreover, it would also be desirable to have better control over all the values related to their reaction with air, namely the energies and the durations of this reaction.
From a practical viewpoint, it would also be appropriate to develop a process for the manufacture of such compounds which is simpler and faster than those which currently exist.
Some intercalation compounds of graphite, namely those of heavy alkali metals, have been known since 1926 [K. Fredenhagen and G. Cadenbach, Z. anorg. allgem. Chemie (1926) 158].
Finally, the description of the state of the art can be completed by the citation of the document U.S. Pat. No. 3,160,670, which teaches the use of KC.sub.8 as a catalyst for chemical reactions.